The invention relates to a process and the device for carrying out the process for producing high-viscosity or highly stabilized, reaction-stable polyamides, in particular polyamide 6, with reactive amino end groups of less than 50 .mu.eq/g and for continuously demonomerizing polyamides.
High-viscosity polyamide can be produced batchwise in autoclaves and continuously in polymerization reactors. However, only a maximum solution viscosity of 3.4 can be achieved by means of these processes (solution viscosity based on polymer-sulfuric acid solution containing 1 g of polyamide in 100 ml of 96% strength acid). However, there are also processes in which polyamide granules are subjected to postcondensation below the polyamide melting point with nitrogen at temperatures in the range of 140-190.degree. C. and with dwell times of 20-60 h. These processes have the disadvantage that the establishment of high viscosities takes a very long time and such processes are therefore uneconomical. Patent PE 3923061 describes a process of this type for polyamide 6 postcondensation, in which the polyamide granules are treated in a heating zone over 35 h.
It is known that, during the polycondensation of .epsilon.-caprolactam to give a polyamide at the reaction temperature of 240-280.degree. C. usually used in production, an equilibrium which results in a water-soluble fraction of about 10-13% is established. The equilibrium is temperature-dependent; the monomer content increases with increasing temperature. Copolyamides, one of whose starting materials is .epsilon.-caprolactam, likewise have a monomer content in chemical equilibrium. For the processing of the granules, it is necessary to reduce the monomer content. The following methods are available for this purpose:
extraction of the granules with hot water and those for PA1 demonomerizing the melt in vacuo.
The aqueous extraction predominantly used today is technically complicated and expensive since it is necessary to carry out the steps of spinning and cooling of the polyamide 6 melt in tape form, comminuting of the tapes to give granules, extraction of the granules, drying of the granules and reconversion of the granules to the molten state. This method of extraction with hot water can be carried out batchwise and continuously. It is used on a large industrial scale by all polyamide 6 producers.
In contrast, the route via the granules can be dispensed with in vacuum demonomerization. The melt arriving from the polycondensation reactor passes continuously through the demonomerization stage and is then fed directly to the spinning means.
U.S. Pat. No. 3,578,640 describes a process for demonomerizing the PA melt by means of a vacuum. In this process, after the vacuum stage, a very complicated technical procedure is carried out for discharging the melt and for the downstream finisher for further reducing the content of extractables. In this technique, for example, shaft seals inevitably lead to leaks on the vacuum system and associated oxidative damage to the melt by atmospheric oxygen which has penetrated. The oxidative damage cannot be suppressed even by feeding in inert gases, in particular nitrogen.